Unable to Activate

Go back to the "someone" that you bought the phone from and have them prepare the phone for sale correctly. This is what they need to do:

What to do before you sell, give away, or trade in your iPhone or iPad - Apple Support

Please look at this --> If you can't activate your iPhone - Apple Support

Carrier Lock:- Request your iPhone’s original Carrier to unlock your iPhone and if they agree and do it, then you may know the further process here --> How to unlock your iPhone for use with a different carrier ...

If you are the owner or the owner has shared all information with you then, in order to open the Activation Lock:- read here --> Turn off Activation Lock - Apple Support Activation Lock for iPhone, iPad, and iPod touch - Apple Support

If you have an unknown Apple ID on the App Store and Apple ID page. (See the pic below)

Learn how to remove a device from a previous owner's account.

If you are not the owner and do not have the Apple ID and Password then there is no way that you can get into this iPhone unless the original owner shares the Apple ID and password with you. If you are lucky you can get into this iPhone if they (owner) accidentally remove the device from their Apple ID.

Oliv12 Said:

"Unable to Activate: I buy an iPhone 7plus from someone. and I erase everything on the iPhone, I tried Activating the iPhone and it keeps on telling me (UNABLE TO ACTIVATE) An update is required) to activate iPhone. Learn more to apple.com/uk/support."

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Getting into a Logged-in Device:

A. Request a Refund:

Seems you've been scammed, though a full-assumption on my part. So, ask this person for a refund. This is the site they should have used prior to handing it down to you: What to do Before you Sell, Give Away, or Trade in your iPhone, iPad, or iPod touch - Apple Support. If they don't log out of it themselves, then you'll never be able to log into it on your own account. They should have known better.

B. Report this:

Where was this purchased? If from a Web site (i.e. eBay or Amazon), then contact the site, seeing if the seller can be held accountable, and if you can get your money back.

C. Learn from This:

Next time you go about purchasing a device from someone, request that they let you log into your own iCloud account on it, prior to handing over the money. If they decide to disabide by those rules you've set forth, then do not proceed with the transaction.